1. Field of the Invention
The present invention relates to an apparatus and method for elevator door/gate control.
2. Brief Description of the Prior Art
Elevator doors are used in passenger elevators and in freight elevators. Passenger elevator doors open horizontally and are lighter in weight than freight elevator doors. A typical passenger elevator door will weigh approximately one hundred pounds compared to freight elevator doors that typically weigh about one thousand pounds. Passenger elevator doors are driven with a cable attached to a sprocket, whereas freight elevator doors are operated with a leaf chain over an H-sheave. Leaf chains are required for freight elevator doors because they can bear greater loads than sprocket driven chains or belts.
A sprocket driven chain or belt such as used with passenger elevator doors provides a direct correlation between the rotation of the sprocket and the linear displacement of the elevator door. A leaf chain, however, slips on the sheave and provides no definite relationship between the rotation of the sheave and linear displacement of the elevator door.
Freight elevators typically include an elevator door and a gate panel. The elevator door functions independently of the gate panel. Conventional freight elevator door/gate control systems are primarily mechanical in nature. The opening and closing of the freight elevator door is controlled with mechanical limit switches. The mechanical limit switches control a motor for opening and closing the door depending upon the path position of the freight elevator door. Typically, freight elevator doors are slowed down as they approach their mechanical limits of fully-open or fully closed. For example, the doors may be run at full speed until the doors physically contact the limit switch, whereupon a direct current is then applied to the motor to assist in braking the elevator door speed. When a two-speed motor is combined with a limit switch, the doors are run at full speed until the limit switch is triggered, then the motor is reduced to half-speed. When the doors are being opened, the doors stop when they hit a mechanical sill. When the doors are being closed, they stop when they slam together. The doors cannot be reduced to zero speed as they are carried by momentum after they trip the limit switch. Limit switches must be periodically repositioned because friction between the elevator doors and track increases with time causing the doors to open and close improperly.
A freight elevator gate panel typically includes a rotary limit switch connected to a roller chain driven by a sprocket. The chain is connected to a gate. The gate position of fully open and fully closed is controlled by the limit switch.
U.S. Pat. No. 5,587,565 describes a passenger elevator door controller that receives pulses generated by an incremental encoder based on the rotation of the chain or belt sprocket drive shaft whenever the passenger elevator door is in motion. During initialization of the system, the controller is taught the number of pulses for travel in each direction. If the doors are set in motion and the power goes off, the encoder will stop counting but the doors will continue drift before they come to a stop. The controller therefore loses track of door position even if there is a battery backup. The system is also susceptible to external electrical noise interfering with the count such as from fluorescent lights and electrical machinery. Because the door position is not monitored in real-time, no adjustments can be made while in motion. Another drawback is the encoder will not update its pulse count if the door is manually moved. Therefore, when the door opens or closes, it will travel the full number of pulses programmed and cause damage to the door.